1. Field of the Invention
This invention is directed to garden string trimmers, and more particularly, to a cutting head for string trimmers which can be easily re-strung.
2. Description of the Prior Art
String trimmers are well known in the prior art. The typical prior art string trimmer comprises a planar cutting head with at least two radially extending strings which is rotated at a sufficient velocity to cause the strings to stiffen. The strings are formed usually from plastic and are sufficiently strong enough to cut through blades of grass, hedges, and other vegetation in a stiffened state. The strings, however, will break upon hitting stone, concrete or some other hard material commonly found in a garden setting.
String trimmers are most often used to cut vegetation along a border of a flower bed or plot of grass which is adjacent to a sidewalk, driveway, or solid structure. Since the cutting head rotates at a speed that makes the strings nearly imperceptible, an operator of the string trimmer is not always able to see that the strings may come into contact with a hard surface and react accordingly. As a result, strings on a string trimmer are commonly broken. Prior art designs of string trimmer cutting heads have attempted to ease the re-stringing process which may be quite time consuming. One type of cutter head design includes an array of grooves and cut-outs in the hub of the cutting head which act to anchor a string therein as in U.S. Pat. No. 4,190,954 to Walto entitled "CUTTING HEAD" which issued Mar. 4, 1980, U.S. Pat. No. 4,905,465 to Jones, entitled "ROTARY CUTTING MEMBER FOR USE WITH LAWN MOWERS AND THE LIKE" which issued on Mar. 6, 1990, and U.S. Pat. No. 5,023,998 to Masciarella, et al. entitled "FILAMENT HOLDER FOR A ROTATING FILAMENT VEGETATION CUTTER" which issued Jun. 18, 1991. This type of cutting head has several drawbacks. First, the grooves and cut-outs include a number of sharp corners which, in time, fray, weaken and break the string disposed therein. Second, a relatively thin string can only be used due to the numerous twists and turns the string must pass through. As a result, the thinner string breaks more often than thicker counter parts and requires repeated replacement. Third, disassembly of the cutter head is required to re-string the cutter head with subsequent bending and manipulation of the string to conform with the grooves and cut-outs formed therein. A typical prior art cutting head requires 1-5 minutes for disassembling and re-stringing. For a commercial gardener who spends eight to ten hours a day using a string trimmer, an hour or more may be wasted for re-stringing.
A second type of string trimmer cutting head design includes a spool of string enclosed within the cutting head, where the string is paid out through peripheral apertures formed in the circumference of the cutting head as required, as disclosed by U.S. Pat. No. 3,708,967 to Geist, et al. entitled "ROTARY CUTTING ASSEMBLY" which issued Jan. 9, 1973, U.S. Pat. No. 4,301,642 to Thurber entitled "SAFETY ROTOR FOR MOWER" which issued Nov. 24, 1981, and U.S. Pat. No. 4,335,510 to Close, et al. entitled "STRING TRIMMER" which issued Jun. 22, 1982. The string is clamped within the cutting head to prevent unwanted extension therefrom. This cutting head design also has several drawbacks. A failure near an aperture may cause the end of the string to retract within the cutting head, thus requiring disassembly of the cutter head and re-threading of the string through the aperture. Although the cutting head may be designed to be readily removed, the common occurrence of a string failing adds up to a significant amount of time spent in disassembling the cutter head. Also, the operator of the string trimmer must carry the weight of an entire spool during the course of operation, which for a commercial gardener may be a substantial time and even an entire work day, resulting in a strenuous effort.
To overcome some of the shortcomings of the prior art, a third type of string trimmer cutter head design avoids the need for disassembly of the cutter head to accomplish re-stringing. For example, U.S. Pat. No. 4,756,146 to Rouse entitled "STRING TRIMMER HEAD AND METHOD" which issued Jul. 12, 1988 includes four arcuate channels, each extending between two points on the periphery of the cutting head. The channels are disposed in pairs so that each pair of channels frictionally engages a string. The string is secured to the cutter head with one end of the string being fed through each channel from a point external to the cutting head and with the ends being pulled through the channels so that the intermediate portion of the string comes into contact with a portion of the periphery of the cutting head disposed between the channels. Another example of the third type of cutting head design is disclosed in U.S. Pat. No. 5,398,416 to Mackey entitled "UNIVERSAL STRING TRIMMER REPLACEMENT HEAD" which issued Mar. 21, 1995 and which discloses a cylindrical block formed with two non-parallel channels. A string is secured in the Mackey cutting head through a series of bends with the string passing through the channels and engaging portions of the outer surface of the cutting head. Although these references avoid the requirement of disassembling a cutter head during the re-stringing process, the required number of bends and threading disclosed in these references is also time-consuming. Due to the shortcomings of the prior art, there is a need for a string trimmer cutting head which may be quickly re-strung.
It is an object of this invention to provide a string trimmer cutting head which is easily and quickly re-strung.
It is also an object of this invention to provide a string trimmer cutting head which can be easily and quickly strung with any gauge string.
It is a further object of this invention to provide a string trimmer cutting head with a centrifugally-driven clamping mechanism for holding the string within the cutting head which allows easy re-stringing of the cutting head.
It is yet another object of this invention to provide a lightweight string trimmer cutting head which contains an amount of string needed only for operation.
It is also another object of this invention to provide a string trimmer cutting head which can be re-strung without disassembly or bending and twisting of the string within the cutting head.